OP: I, too, am starting my SoPs. For what it's worth, fuzzylogician and jacib are echoing everything I've been told by profs and graduate friends.
If it helps, some even offered some strict guidelines:
If it says 1000+ words, do 1000.
If it says 1000 words, do 750.
If it says 2-3 pages, do 2.
If it says 500 words, you better find the 500 best words on that messy Word doc!
That is, include only the most relevantly titillating information; I think it's a general rule that SoPs, especially the shorter ones, are largely used to judge an applicant's ability to whittle a surplus of information (like "your life") into its most pertinent essentials, and to present those essentials with insightful brevity. Anyone can do research, but few can identify and present their most salient findings in a concise and engaging manner, and it's that skill that'll help your app climb toward the top of the stack. Additionally, AdComs will certainly be reading loads of SoPs that not only reach their limits but even extend past them, so be assured that they'll be grateful to see that you know how to edit yourself.
For those unhelpful schools that didn't indicate any word or page counts, it never hurts to e-mail them. The onus is on them, not you, to set the parameters of the SoP, even if their intention is to see how you perform without guidelines.